Control system for firing black liquor recovery boiler auxiliary fuel in response to plant load swings

ABSTRACT

A black liquor recovery boiler is provided with a plurality of combustion zones for firing black liquor at a relatively constant rate and auxiliary fuel at variable rates. In response to signals indicative of the total paper plant steam demand and the rate of firing of black liquor, the air required for completing the combustion of black liquor and for firing auxiliary fuel is supplied to the boiler. The lesser of the plant steam demand signal and a signal indicative of the actual amount of air available in a selected combustion zone for firing auxiliary fuel operates a device for controlling the pressure of auxiliary fuel supplied to such combustion zone. The combustion air is apportioned to the plurality of combustion zones according to the actual amount of auxiliary fuel and black liquor supplied to each zone.

United States Patent Inventor Roy Roger Herbst Birmingham, Ala.

Appl. No. 62,815

Filed Aug. 11, 1970 Patented Dec. 7, 1971 Assignee The Rust Engineering Company Birmingham, Ala.

CONTROL SYSTEM FOR FIRING BLACK LIQUOR RECOVERY BOILER AUXILIARY FUEL IN RESPONSE TO PLANT LOAD SWINGS 12 Claims, 4 Drawing Figs.

11.8. CI 122/7 C, 122/448 R Int. Cl F22b 1/18 Field of Search 122/7 R, 7 C, 448 R, 479 R References Cited UNITED STATES PATENTS 11/1958 Allen 122/7X Primary Examiner- Kenneth W. Sprague Al!0rneysDavid S. Urey, Alan C. Rose and Alfred B. Levine ABSTRACT: A black liquor recovery boiler is provided with a plurality of combustion zones for firing black liquor at a relatively constant rate and auxiliary fuel at variable rates. In response to signals indicative of the total paper plant steam demand and the rate of firing of black liquor, the air required for completing the combustion of black liquor and for firing auxiliary fuel is supplied to the boiler. The lesser of the plant steam demand signal and a signal indicative of the actual amount of air available in a selected combustion zone for firing auxiliary fuel operates a device for controlling the pressure of auxiliary fuel supplied to such combustion zone. The combustion air is apportioned to the plurality of combustion zones according to the actual amount of auxiliary fuel and black liquor supplied to each zone.

PATENTEI] um 1 I971 ROY ATTORNEY CONTROL SYSTEM FOR EWING BLACK LIQUOR RECOVERY BOILER AUXILIARY lFlUlElL IN RESPONSE TO PLANT LOAD SWINGS BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates to a recovery boiler combustion control system and, more particularly, to a system for automatically controlling the combustion of auxiliary fuel in a black liquor recovery boiler in response to plant load swings.

In the production of pulp, wood chips are cooked in a digester with white liquor. The active chemicals (sodium hydroxide and sodium sulfide) in the white liquor separate the lignin from the cellulose in the wood to provide pulp for paper manufacture. The lignin and spent chemicals form black liquor which is connected in an evaporator and burned in a black liquor recovery boiler. In burning black liquor, the recovery boiler is effective to recover the chemicals for reuse in the digester and to produce a relatively constant amount of heat for steam generation purposes. Because the steam demand of a paper mill may vary over a wide range, and in view of the relatively constant rate of steam generation using black liquor, it is desirable to provide facilities which enable the recovery boiler to respond to variations in the mill steam demand by burning auxiliary fuel.

2. Description of the Prior Art Notwithstanding the importance of recovery boilers in the operation of paper mills, it has been customary in the past to manually control the critical aspects of recovery boiler combustion. The problems faced by boiler engineers in manually controlling recovery boiler operations may be appreciated when it is understood that recovery boilers are operated under extremely variable conditions. For example, even when a recovery boiler is operated as a base loaded boiler using black liquor as the only fuel, the airflow and black liquor flow must be adjusted to maintain a relatively constant steam output because the chemical composition of black liquor is variable. Moreover, when an auxiliary secondary fuel, such as gas, is burned in the boiler to melt down the smelt bed, the flow of primary and secondary air must be regulated according to the amount of both the black liquor and secondary gas inputs.

In addition, to increase the steam output of a recovery boiler, a secondary fuel, such as gas, may be burned in the boiler. This adds another variable which has been controlled by the engineer.

In the past, the boiler engineer has been assisted by safety systems which indicate whether or not an auxiliary burner is operating with at least a minimum required airflow. Such systems have not, however, indicated whether or not the proper amount of air is being used to fire the auxiliary fuel. Thus, the control of the total combustion air and fuel requirements of the recovery boiler have been dependent upon and skill and experience of the boiler engineer. Despite the experience and efforts of boiler engineers to closely control recovery boiler operation, accidents have resulted from manual operation of recovery boilers.

In an effort to reduce the hazards accompanying recovery boiler operation, a system was previously developed to (1) measure the total amount of fuel supplied to a recover boiler and (2) control the total flow of combustion air to selected zones of the boiler in response to the total amount of fuel measured. This system is the subject of patent application, Ser. No. 845,353 filed in the US. on July 28, I969 by the assignee of the present application. While such a system provided a satisfactory altemative-to manual control of recovery boiler firing, the system was not automatically responsive to variations in the plant steam demand. Rather, the black liquor and auxiliary fuel inputs were manually set at desired levels and the system operated automatically to control the flow of combustion air according to the total fuel input. Further, the prior system permitted the operator to increase auxiliary fuel flow before sufficient combustion air was available in the combustion zones.

SUMMARY OF THE INVENTION Research has been conducted in an endeavor to reduce the hazards of recovery boiler operation while rendering the system production of such boilers responsive to plant load swings. Such research indicates that the normal firing of black liquor can remain relatively constant while the rate of firing of auxiliary fuel is varied automatically, within limits, in response to plant load swings. In the event the plant steam demand increases while auxiliary fuel is being fired, an auxiliary fuel demand signal indicative of total plant steam demand is utilized to increase the total flow of combustion air. At the same time, the auxiliary fuel demand signal is compared to a signal indicative of the air available in the combustion zones. When sufficient combustion air is available for firing auxiliary fuel, the position of appropriate valves is varied to supply additional auxiliary fuel to auxiliary fuel burners in the combustion zones. The pressure of auxiliary fuel supplied to the burners is compared to the burners minimum operating pressure to provide at least the minimum pressure for stable combustion.

Further, the flow of air to a particular combustion zone is related to the actual flow of auxiliary fuel to such zone so that the total flow of combustion air is distributed as required for proper combustion.

An object of the present invention is to provide a new and improved recovery boiler combustion control system.

A further object of this invention resides in a system for automatically controlling the combustion of auxiliary fuel in a black liquor recovery boiler in response to plant load swings.

Another firing object of the present invention is to provide automatic facilities for firing black liquor at a relatively constant rate while firing auxiliary fuel a a rate determined by the plant steam demand.

Still another object of the present invention resides in the automatic supply of auxiliary fuel to burners of a recovery boiler wherein the pressure of such fuel supplied is prevented from falling below a minimum tumdown pressure.

A still further object of the present invention resides in the provision of facilities for automatically controlling the flow of combustion air to a black liquor recovery boiler in response to the total plant steam demand.

An additional object of this invention is to provide facilities for selecting, as an auxiliary fuel pressure control signal, the lower of a combustion zone air available signal and a gas demand signal which is indicative of the total plant steam demand.

DESCRIPTION OF THE DRAWINGS placed side by side with lines AA and BB thereon aligned,

form FIG. 2, which is a schematic diagram of a first embodiment of the control system of the present invention; and

FIG. 3 is a schematic drawing of a second embodiment of the control system of the present invention, wherein the drawing of the second embodiment is illustrated for use in conjunction with FIG. 28 by substituting FIG. 3 for that portion of FIG. 2A which is between dashed lines C-D in FIG. 2A.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings wherein like reference characters are used throughout to designate like elements, an illustrative and preferred embodiment of the present invention is shown in FIG. 1 including a chemical recovery unit, such as a boiler 10. The boiler 10 extends vertically from a lower section 12 for a height of up to 15 stories, for example, and is provided with a reduction zone A at the lower end thereof and oxidation zones B and C spaced upwardly from the reduction zone A. The zones A, B and C are in open communication relative to each other to facilitate the upward flow of products of combustion to an exhaust stack 14 under the operation of an induced draft fan 15.

The walls of the boiler are lined with steam generating tubes 16 of a boiler heat exchange system which includes additional heat exchanger tubes 18 located at the upper end of the boiler 10. A mixture of steam and water (at saturation temperature for the particular pressure at which the boiler unit is operated) flows upwardly through the tubes 16 and 18 to a boiler header 20. The boiler 10 may be operated to produce steam at a pressure of 875 lbs. per sq. inch and superheated to a desired value, such as 825 F. The steam is supplied from the header 20 to a plant master header 21 (FIG. 2) from which it is distributed to desired points of use within the paper mill.

In the normal operation of the boiler 10, black liquor which has been evaporated in the Kraft process to a desired density (such as a 50-65 percent solids content), is sprayed into the boiler 10 through nozzles 22 at a suitable pressure and temperature. Thus, the boiler 10 is commonly referred to as a black liquor recovery boiler." Most of the sprayed liquor particles fall to the bottom 23 of the boiler 10 in counterflow to the products of combustion rising from the reduction zone A. The falling particles are dried and fonn a pile or smelt bed 24. Preheated primary air is supplied to the reduction zone A through primary wind boxes 25 in amounts sufficient to produce a reducing atmosphere or condition therein. The residual chemicals from the black liquor flow through a spout 26 to a receiving tank 27 for reuse in the Kraft process. The combustible materials in the products of combustion which rise from the smelt bed 24 are burned with preheated secondary and tertiary air. The secondary air is supplied to the boiler 10 through secondary wind boxes 38 whereas the tertiary air is supplied to the oxidation zone C through tertiary wind boxes 29. The tertiary air establishes a turbulent condition in the zone C to assure complete combustion of the products of combustion. The temperature in the reduction zone A is generally in the range of 2,l002,200C F., whereas the products of combustion leave the combustion zone C at about l,700 F.

The total flow of combustion air to the boiler 10 is provided by primary and secondary forced draft fans 30 and 31, respectively. The fans 30 and 31 have vanes 32 and 32 respectively which are effective to vary the total airflow at constant blower speed. The first fan 30 supplies primary combustion air through a duct 34 to a primary air manifold 35 which supplies air to the primary wind boxes 25 spaced circumferentially around the reduction zone A.

The secondary fan 31 supplied combustion air through a duct 36 to a secondary air manifold 37 and a tertiary air manifold 38. The flow of secondary air to the secondary wind boxes 28 is controlled by vanes 39 in the manifold 37, whereas the flow of tertiary air to tertiary wind boxes 29 is controlled by vanes 40.

While the boiler 10 may operate with black liquor as the only fuel supplied, auxiliary fuel may be supplied in response to plant load swings. Various auxiliary fuels, such as fuel oil, may be used. However, for purpose of illustration, the description herein refers to gas as the auxiliary fuel. According to the control system shown in FIG. 2, the gas may be supplied through nozzles 42 and 43 of respective secondary and tertiary burners 44 and 45.

The primary air supplied to the reduction zone A is insufficient to completely combust the black liquor supplied through the nozzles 22. As shown in FIG. 2, the vanes 33 of the second forced draft fan 31 are automatically controlled so as to provide air in the secondary air manifold 37 sufficient to complete the combustion of the black liquor to fully combust the secondary gas supplied to the secondary burner 44, to supply air to the tertiary wind box 29 sufficient to assure full combustion of any black liquor which was not fully burned in zone B, as well as for complete combustion of the tertiary gas supplied to the tertiary burner 45.

In the past, the supply of fuels and air to recovery boilers has been controlled manually. The difficulties of such manual control are apparent from the fact that 22 black liquor recovery boiler explosions have been attributed to problems in manually controlling such boilers during the firing of auxiliary fuel.

More recently, an automatic system has been developed for controlling the amount of combustion air which is supplied to the combustion zones of a recovery boiler. In such systems, the amount of fuel actually supplied to each combustion zone is sensed and is used as the basis for controlling the amount of combustion air supplied.

Notwithstanding the substantial improvement in safety provided by the aforementioned automatic system, additional developments resulting in the system of the present invention indicate that power plants which generate steam for use in paper mills may be more responsive to plant load swings when the black liquor recovery boiler is capable of automatically firing auxiliary fuel in response to plant load swings.

Referring to FIG. 2, the present invention may be understood in reference to a primary embodiment thereof which includes a control system for regulating the operation of the recovery boiler 10. The initial on startup operation of the boiler 10 may be controlled by a startup circuit such as that disclosed in the aforementioned application, Ser. No. 845,358.

In the operation of the boiler 10 as a base-loaded boiler, a black liquor flow controller 51 is set manually for generating a black liquor demand signal 52. The signal 52 is applied to a first comparison circuit 53 which compares the signal 52 to a signal 54 which is indicative of the actual black liquor flow rate through the nozzles 22. The magnetic field in a flow meter 56 is varied according to the velocity of black liquor therethrough to generate the signal 54. The signal 54 from the flow meter 56 may be in millivolts, for example, for application to the comparison circuit 53.

The first comparison circuit 53 may be an integral operational amplifier having a selected reset rate and a potentiometer (not shown) in the signal input circuit. The reset rate and gain are based upon boiler response so that the boiler control is stable. The circuit 53 compares the signals 52 and 54 to produce a black liquor flow control signal 57 which is used to control the flow rate of black liquor, such as by positioning a valve (not shown) or by regulating a black liquor pump 55.

The black liquor demand signal 52 is also applied to a multiplier 58. A black liquor-to-air ratio controller 59 may be adjusted according to the quality of the black liquor to generate a black liquor-to-air ratio signal 61 which is also applied to the multiplier 58. In response to the signals 52 and 61, the multiplier 58 generates a primary air demand signal 62 in accordance with the amount of primary air necessary to partially burn the black liquor in the reduction zone A.

The actual flow of primary air in the primary air manifold 35 is measured by a circuit 64 to produce an actual primary airflow signal 66, which, after processing in a standard manner, is indicative of and varies linearly with the flow of primary air to the reduction zone A. The primary air signals 62 and 66 are applied to a second comparison circuit 67 which produces a primary air control signal 68. When the actual primary airflow is less than that required by the primary air demand signal 62, the primary air control signal 68 will operate a transducer 69 for adjusting the position of the primary air vanes 32. The vanes 32 are adjusted until the amount of primary air admitted to the primary forced draft fan 30 equals the amount required by the primary air demand signal 62.

Because the reduction zone A is supplied with less air than is required for complete combustion of the black liquor, excess air is supplied to the combustion zone B under the control of the black liquor demand signal 52. In particular, in response to the signal 52, a first summing circuit 72 generates a secondary and tertiary air control 73 which is converted to linear motion by a transducer 74 to adjust the position of the vanes 33 at the inlet to the second forced draft fan 31 to supply excess air to the combustion zones B and C.

As indicated above, the boiler 19 may be operated as a base-loaded boiler using black liquor as the only fuel. However, in the event the supply of black liquor is insufficient to meet the base-load B.t.u. requirements, auxiliary fuel, such as gas, may be applied to make up the B.t.u. deficiency of the black liquor. In such event, the total amount of gas supplied to the secondary and tertiary burners is controlled by the control system shown in FIG. In particular, the flow rate of gas supplied is sensed by an orifice plate 81 mounted in a main gas header 76 which supplies gas to the secondary and tertiary burners 44 and 45 respectively. The orifice plate 81 operates a measuring circuit 82 which produces an actual total gas flow signal 83 which is applied to a mode selector switch 94. With the switch 84 in a base-load" position, the total gas fiow signal 93 is connected to the first summing circuit 72 for addition to the black liquor demand signal 52. In this manner, the secondary and tertiary air control signal 73 is indicative of the air required to completely combust both the black liquor and base-loading gas.

Alternatively, the mode selector switch 84 may be moved to a plant-steam demand" position to condition the boiler for response to changes in the steam demand of the paper mill. Such changes in steam demand may be relatively frequent and can be of wide range (in terms of B.t.u. requirement). Accordingly, the steam demand changes are commonly referred to as plant load swings.

In prior recovery boiler combustion control systems, attempts have been made to respond to plant load swings by varying the rate at which black liquor is fired. However, such systems depend, of course, upon the availability of additional quantities of black liquor and also present control problems in view of the inherently variable B.t.u. content and unstable combustion properties of black liquor. It may be appreciated, therefore, that the present system avoids such prior limitations in that it responds directly to plant load swings by firing auxiliary fuel rather than black liquor.

In particular, and referring again to FIG. 2, the mode selector switch 84 is set in the plant-steam demand" position to condition the recovery boiler to plant load swings which are apparent in the plant master header 2ll. The steam pressure in the plant master header 21 is sensed by a master steam demand controller 91. The controller 91 is provided with gain and reset proportional to the overall plant response for producing a plant steam demand signal 92 in proportion to total plant steam demand. The plant steam demand signal 92 is applied to the other boilers (not shown) of the paper plant by a conductor 92A and to a recovery boiler master controller switch 93.

With such gain and reset (or tuning), the controller 9i produces the signal 92 with the least time delay. As a result, all of the boilers in the plant can quickly respond to variations in the steam pressure so that deviations from the desired plant operating pressure are minimized and stable control of the boilers is maintained. It may be appreciated that such quick response to plant steam pressure variations is essential because most processes operate at maximum efficiency using steam supplied at a specific pressure and temperature.

The switch 93 may be set in a manual position so that the recovery boiler does not respond to the signal 92. On the other hand, with the switch 93 in an automatic" position, the signal 92 is applied to the boiler control system in the form of a gas demand signal 94. It may be appreciated that all of the plant boilers, including the recovery boiler, respond to the plant steam demand signal 92.

In the system of the present invention, the gas demand signal 94 is applied to the mode selector switch 94 which conducts it to the first summing circuit 74. The circuit 72 generates the secondary and tertiary air control signal 73 according to the sum of the gas demand signal 94 and the black liquor demand signal 52. In response to the air control signal 73, the transducer 74 adjusts the opening of the inlet vanes 33 to the second forced draft fan to provide the air necessary for complete combustion of the base-load black liquor and the auxiliary gas.

In particular, because the gas demand signal 94 is representative of the total plant steam demand, a demand which includes the demand provided for by the burning of black liquor, the summing of the signals 94 and 52 will result in the supply of secondary and tertiary air in excess of that required to burn the black liquor. Significantly however, the use of the gas demand signal 94 in this manner enables the boiler to fire black liquor at a preselected rate despite the occurrence of wide plant load swings. For example, the plant steam demand may be reduced to such a low level that only the firing of black liquor is necessary to satisfy the demand. In this event, the firing of black liquor continues without interruption. Thus, the firing of black liquor is limited only by the availability of black liquor.

At the same time as the gas demand signal 94 is applied to he mode selector switch 84, the gas demand signal 94 is applied to a secondary low select circuit 102. The circuit 101 is also responsive to a secondary air available signal 102 which is generated by a secondary difference circuit 103. The secondary difference circuit 193 is calibrated such that under baseload conditions (with no gas demand signal 94), it does not generate the secondary air available signal I02. That is, when only enough secondary air is supplied to the secondary air manifold to fire black liquorsuch that no excess air is available to burn the auxiliary fuelthe secondary air available signal 1102 is not generated.

In greater detail, the actual secondary airflow in the secondary air manifold is measured by an orifice plate 104 of a measuring circuit I06 and an actual secondary air flow signal 107 is applied to the secondary difierence circuit 103. Also, the actual primary airflow signal 66 is applied to the secondary difference circuit I03. Under baseload conditions, the signals I07 and 66 will be related according to the ratio of secondary to primary air required to combat the black liquor. When such ratio exists, the secondary air available signal I02 will be zero. However, when such ratio is exceeded, there is excess air supplied to the secondary air manifold and the secondary air available signal 1192 will be generated to indicate that secondary gas can be burned according to the amount of such excess air.

The secondary low select circuit 101 selects the lower of the two signals 94 and 102 and applies the lower signal 111 to a secondary gas summing circuit 112. The circuit 112 responds to the sum of the signal 111 and a secondary feedback signal M3 to generate a secondary gas control signal 116 which operates a secondary gas valve lll7 to control the pressure of secondary gas supplied to the secondary burners 44. The valve llI7 may be positioned automatically to supply secondary gas at pressures between a minimum gas pressure P and a maximum gas pressure.

The secondary burners 44 may be of the type which operate with a stable flame only if the pressure of the gas supplied thereto is at least equal to the minimum pressure P In such event, the secondary feedback signal I13 may be generated in the following manner to maintain at least the minimum pressure P at the secondary burners 44. The gas pressure in the secondary gas burners 44 is measured by a sensor 121 of a measuring circuit 122 to produce a first feedback signal 123 which is applied to a turndown comparator 124. In the event the feedback signal I23 indicates pressure which is less than the minimum pressure permitted for the secondary burners, the comparator H24 applies the secondary feedback signal 113 to the secondary gas summing circuit 112 such that the secondary gas control signal 116 will adjust the secondary gas valve M7 for operation of the secondary burners 44 at the minimum pressure.

The supply of tertiary gas to the tertiary burners 45 is controlled in a manner which is similar to that of the supply of secondary gas to the secondary burners 44. In greater detail, the actual flow of tertiary air in the tertiary air manifold 38 is measured by an orifice plate I31 which controls a measuring circuit 1132 for generating a signal 133 in the proportion to the tertiary airflow. The tertiary airflow signal 133 is applied to a tertiary difference circuit 134 along with the actual primary air signal 66. The circuit 134 is calibrated similar to the secondary difference circuit 103 in that there is no output when the ratio of the tertiary air signal to the primary air signal is only sufficient to burn black liquor. When the ratio is exceeded, that is, when tertiary air in excess of that needed to completely combust the black liquor is supplied, the tertiary difference circuit 134 generates a signal 136 indicative of the tertiary air available to combust tertiary gas. The tertiary air available signal 136 and the gas demand signal 94 are applied to a tertiary low select circuit 137 which applies the lower signal 138 of the two signals 94 and 136 to a tertiary gas summing circuit 141. The circuit 141 sums a tertiary feedback signal 142 and the lower signal 138 to produce a tertiary gas control signal 143 which is applied to a tertiary gas valve 145 to control the pressure of tertiary gas in the tertiary gas burners 45. A sensor 144 of a measuring circuit 146 monitors the tertiary gas pressure in the tertiary gas manifold and generates a second feedback signal 147 which is applied to a tertiary tumdown comparator 148. In the event the second feedback signal 147 indicates tertiary gas pressure which is less than the minimum pressure permitted for the tertiary burners, the comparator 148 generates the tertiary gas feedback signal 142 to the extent that the tertiary gas pressure is less than the minimum pressure of the tertiary burners 45. In this manner, the tertiary burners 45 will be maintained at a level of operation which will promote stable combustion of the tertiary gas.

As described above, the flow of secondary and tertiary air into the second forced draft fan 31 is controlled according to the black liquor and plant steam demands whereas the secondary and tertiary gas pressure is controlled according to both the plant steam demand and the excess secondary and tertiary air available. In addition, the vanes 39 and 40 in the respective secondary and tertiary air manifolds 37 and 38 are controlled in response to the respective flow of secondary gas, tertiary gas and black liquor. More particularly, the actual primary air flow signal 66 is applied to a bias circuit 153. When black liquor is being burned in the boiler, the bias circuit 153 is adjusted to vary the ratio of primary to secondary and tertiary air according to the solids content of the black liquor. That is, if the solids content increases, then the bias circuit 153 is adjusted to generate an increased output signal 154, whereas when the solids content decreases, the bias circuit is adjusted to generate a decreased output signal 154. The output signal 154 and the secondary air flow signal 107 are applied to a first algebraic summing circuit 156. The circuit 156 generates a secondary signal 157 which is indicative of the amount of air available for burning secondary gas. A third comparison circuit 164 compares the signal 157 to a secondary gas flow signal 158 which is indicative of the amount of air required to burn the secondary gas actually being supplied to the burners 44. The signal 158 is produced by a circuit 161 which measures the flow of secondary gas to the secondary gas burner 44. A manual controller 162 is provided for adjusting the gas to air ratio of a secondary gas multiplier 163 so that the secondary gas flow signal 158 can be varied to account for desired variation of the secondary gas to air ratio.

To the extent that the secondary gas flow signal 158 exceeds the secondary signal 157 (indicating a greater amount of secondary air required to burn the secondary gas), the third comparison circuit 164 generates a damper control signal 166 which is applied to a totalizer 171. The totalizer 171 adds the damper control signal 166 to the primary air demand signal 62 and produces a signal 172 which is indicative of the total amount of secondary air required to burn black liquor and secondary gas. The signal 172 is applied to a fourth comparison circuit 173.

The actual secondary airflow signal 107 is applied to a function generator 176 which varies the secondary airflow signal 107 according to the desired secondary burner fuel to air ratio. The secondary airflow signal 107 is then applied to the fourth comparison circuit 173. The circuit 173 compares the signal 172 to the signal 107 and operates a transducer 177 which is effective to open the vanes 39 in the secondary air manifold 37 in the event the totalizer signal 172 exceeds the secondary airflow signal 107 so that an increased flow of secondary air is admitted to the secondary wind boxes 28.

To control the vanes 40 in the tertiary air manifold 29, the output signal 154 from the bias circuit 153 is applied to a second algebraic summing circuit 181. The tertiary airflow signal 133 is also applied to the circuit 181 which generates a tertiary signal 182 which is indicative of the amount of tertiary air available to burn tertiary gas. A circuit 183, including orifice plate 184 mounted in the tertiary gas manifold 29, measures the flow of tertiary gas and produces a signal 186 indicative of the actual flow of tertiary gas. The signal 186 is applied to a multiplier 187 which receives a tertiary gas to air ratio signal 188 from a controller 189. The multiplier 187 produces a signal 191 indicative of the tertiary air required to completely combust the actual flow of tertiary gas. The signals 182 and 191 are applied to a fifth comparison circuit 192 which generates a control signal 193 in accordance with the difference between the signals 182 and 191. The control signal 193 and the black liquor demand signal 52 are applied to a fifth summing circuit 196 which generates a tertiary damper control signal 197. The signal 197 operates a transducer 198 which controls the position of the vanes 40 in the tertiary air manifold 29 so that the tertiary air required to completely combust the black liquor and the tertiary gas is supplied to the oxidation zone C.

As an illustration of how the control system of the present invention may be adapted to an exiting manually controlled recovery boiler, reference is made to FIG. 3. The boiler in this example is similar to the boiler 10 in that it has three combustion zones A, B and C and the accompanying primary, secondary and tertiary auxiliary (gas) fuel and air supplies. However, the secondary gas supply manifold of this recovery boiler cannot accommodate an orifice plate such as the plate 184 shown in FIG. 2. Thus, the secondary gas flow cannot be measured. Also, a majority of the gas burners 44 are located in the secondary zone B.

With these characteristics of the exemplary boiler in mind, it may be understood from FIG. 3 that the control system shown in FIG. 2 has been modified in the following manner. Assuming the boiler has been firing gas and that there is an increase in the plant steam demand, an increased gas demand signal 94 is applied to the totalizer 171 rather than to the mode selector switch 84 (which is eliminated). The totalizer 171 adds the primary air demand signal 62 and the gas demand signal 94 to produce a signal 201 which is indicative of the secondary air required to burn the black liquor and the secondary gas. A fourth comparison circuit 199 compares the signal 201 to the actual secondary air flow signal 107 such that the transducer 177 is caused to open the vanes 39 wider. With the vanes 39 open wider, more secondary air is supplied to the secondary wind boxes 28.

The increased flow of secondary air is sensed by the orifice plate 104 and a greater actual secondary air flow signal 107 is generated. The greater signal 107 causes the secondary difference circuit 103 to generate an increased secondary air available signal 102 which opens the secondary gas valve 117. With the valve 117 open wider, the boiler burns more secondary gas in response to the increased plant steam demand.

The increased secondary gas supply is sensed by the orifice plate 81 and the resulting actual total gas flow signal 83 is applied to the multiplier 163 (rather than to the mode selector switch 84). The multiplier 163 responds to the controller 162 and varies the signal 83 according to the desired gas to air ratio so that the signal 83 is indicative of the total air necessary to burn the total gas. The third comparison circuit 164 compares the signal 83 to the secondary signal 157 and produces a secondary and tertiary air control signal 202 to the extent and in the direction that the signals 83 and 157 are unequal.

The signals 202 and 52 are added by the first summing circuit 72 which generates a signal 203 which is indicative of the combustion air required to completely combust the secondary and tertiary gas and to complete the combustion of the black liquor. The signal 203 causes the transducer 741 to open the inlet vanes 33 to the second forced draft fan 3! so that the secondary and tertiary air flow rates increase.

Because the primary airflow has not been changed, the secondary and tertiary air available signals 102 and 136, respectively, increase to permit more gas to be burned in response to the gas demand signal 94.

The above described circuits 32 and 183 are responsive to the increased gas flow rates and initiate the appropriate responses until the gas demand signal 94 decreases in response to a decrease in the plant steam demand or the respective secondary and tertiary air available signals 102 and 136 exceed the gas demand signal 94.

It may be appreciated that despite the differences in the recovery boiler of the foregoing example, the control system of the present invention may be adapted to existing recovery boilers to render such boilers capable of firing auxiliary fuel in response to plant load swings.

It is to be understood that the above-described arrangemerits are simply illustrative of the application of the principles of this invention. For example, the sensing devices of the present invention could be arranged to provide inputs to a suitable programmed computer which would operate the valves and vanes of the recovery boiler 10. These and other arrangements may be readily devised by those skilled in the art which will embody the principles of this invention and fall within the spirit and scope thereof.

lclaim:

l. A recovery boiler for generating steam at a rate which varies with variations in a signal indicative of the total plant steam demand, said boiler including a reducing zone and at least one oxidizing zone for firing recoverable fuel and auxiliary fuel, which comprises:

means responsive to the recoverable fuel firing demand for supplying primary air to said reducing zone;

means for supplying said oxidizing zone secondary air in excess of the amount required to completely combust said recoverable fuel to establish an excess air condition therein;

means responsive to said excess air condition for producing a secondary air available signal;

means for supplying said auxiliary fuel to said oxidizing zone at a variable pressure; and

means responsive to the lower of the secondary air available signal and the plant steam demand signal for regulating the pressure of the auxiliary fuel supplied by said auxiliary fuel supplying means to vary the rate of steam generation as the plant steam demand varies.

2. A recovery boiler in accordance with claim 11 in which:

said means responsive to said excess air condition is responsive to at least a preselected difference between the flow of primary air to said reducing zone and the flow of secondary air to said oxidizing zone for producing said secondary air available signal; and

said regulating means includes means for measuring the regulated pressure of said auxiliary fuel, means for producing a feedback signal in response to a decrease of the regulated pressure below a selected minimum pressure, and means for adding the feedback signal and said lower signal to maintain the pressure of the auxiliary fuel at no less than the minimum pressure.

3. A recovery boiler according to claim 1, in which:

said secondary air supply means is responsive to both said demand for firing recoverable fuel and said plant steam demand so that in response to increased plant steam demand additional secondary air is supplied to said oxidizing zone to establish said excess air condition.

4. A recovery boiler according to claim ll, in which a plurality of oxidizing means are provided superadjacent said reducing zone in said boiler;

said secondary air supplying means is responsive to both said plant steam demand and said demand for firing recoverable fuel for establishing said excess air condition in each of said oxidizing zones; and said secondary air available signal producing means and said pressure regulating means are provided for each of said oxidizing zones so that the pressure of auxiliary fuel supplied to each of said oxidizing zones varies as the plant steam demand varies. 5. A system for controlling the combustion of auxiliary fuel in a black liquor recovery boiler, which comprises:

boiler means for burning black liquor and auxiliary fuel,

said means including a reduction zone and at least one oxidizing zone, said means including an air supply for each of said zones; means responsive to the plant steam demand for generating a plant steam demand signal; means for producing a black liquor demand signal indicative of the amount of black liquor supplied to said boiler means; means responsive to the sum of the plant steam demand signal and the black liquor demand signal for controlling the air supply for said oxidizing zone to establish in said oxidizing zone a condition of air in excess of that required to fully combust said black liquor; means responsive to said excess air condition for producing a signal indicative of the amount of said excess air; means for selecting the lower of the plant steam demand signal and said excess air signal to produce an auxiliary fuel control signal; and means responsive to said control signal for regulating the amount of auxiliary fuel supplied to said oxidizing zone. 6. A system according to claim 5, in which: said regulating means includes means for measuring the pressure of the auxiliary fuel supplied to said oxidizing zone, means for comparing the measured pressure to a minimum operating pressure to generate a feedback signal in response to a measured pressure less than said minimum pressure, and means for summing said control signal and said feedback signal to maintain said pressure of auxiliary fuel supplied to said first combustion zone equal to or in excess of said minimum pressure. 7. A recovery boiler combustion control system according to claim 5, in which:

secondary and tertiary oxidizing zones are each provided with conduit means for distributing air from said controlled air supply, each of-said oxidizing zones also being provided with said regulating means for supplying auxiliary fuel to each secondary and tertiary zone; means are provided for each of said regulating means for measuring the amount of auxiliary fuel supplied to each of said oxidizing zones to generate secondary and tertiary auxiliary fuel flow signals; means are provided for each of said conduit means for producing secondary and tertiary airflow signals indicative of the amount of air supplied to each of said oxidizing zones; means are provided responsive to a difference between said secondary airflow signal and said secondary gas flow signal for generating a secondary signal to control the airflow in said secondary zone conduit means; and means responsive to a difference between said tertiary airflow signal and said tertiary gas flow signal are provided for producing a tertiary signal to control the airflow in said tertiary zone conduit means. 8. A system according to claim 5, in which: a secondary oxidizing zone is provided for burning black liquor and secondary auxiliary fuel; a tertiary oxidizing zone is provided for burning black liquor and tertiary auxiliary fuel; said air supply and air supply controlling means provide excess combustion air for both said secondary and tertiary zones; said excess air signal producing means produces a secondary excess air signal indicative of the excess secondary air and a tertiary excess air signal indicative of the excess tertiary air;

one of said selecting means is provided for each of said excess air signals so that secondary and tertiary auxiliary fuel control signals are produced; and

said regulating means includes separate means responsive to respective ones of said auxiliary fuel control signals for regulating the amount of auxiliary fuel supplied to the secondary zone and the tertiary zone. 9. A black liquor recovery boiler having a plurality of combustion zones for firing black liquor at a relatively constant rate and auxiliary fuel at variable rates to generate steam for use in a paper plant having a variable total plant steam demand, which comprises:

means responsive to a first signal indicative of the total plant steam demand and a second signal indicative of the demand for firing black liquor for supplying to said boiler combustion air sufficient to complete the combustion of the black liquor and to fully combust the auxiliary fuel;

supply means for individually feeding the auxiliary fuel to said zones at selected pressures;

means responsive to the lesser of the plant steam demand signal and a signal indicative of the actual amount of air available in each of said zones to combust the auxiliary fuel for regulating said supply means to supply auxiliary fuel to each said zone at a selected pressure; and

means for apportioning said combustion air to said combustion zones according to the actual amount of auxiliary fuel supplied to each of said zones.

10. A boiler for recovering chemicals from black liquor according to claim 9, in which:

said means for supplying secondary air includes means for summing a signal indicative of the demand for primary air and the plant steam demand signal to produce a secondary air demand signal, means responsive to the actual flow of secondary air for producing an actual secondary air signal, and means responsive to the difference between said secondary air demand signal and said actual secondary air signal for supplying said excess secondary air to said oxidizing zone.

ll. In a unit for recovering chemicals from black liquor and for producing steam in response to a plant steam demand signal, said unit including a combustion chamber having a reducing zone and first and second oxidizing zones, means for supplying to said reducing zone black liquor in response to a black liquor demand signal, means for supplying to said reducing zone air required to partially burn said black liquor, means for supplying auxiliary fuel to each of said first and second zones and means for supplying air to said first and second zones; said air supply means having an air inlet, a first zone outlet and a second zone outlet; said unit being included in a plant having a variable steam demand, the improvement in said unit which comprises:

means responsive to the total flow of auxiliary fuel for producing a first signal indicative of the air required to burn the total flow of auxiliary fuel;

means responsive to the flow of air to said first zone and to said reducing zone for producing a second signal indicative of the amount of air available in said second zone to burn auxiliary fuel;

means for summing a signal indicative of the difference between said first and second signals with the black liquor demand signal for controlling said air inlet to make excess air available to said first and second zones;

means responsive to at least a selected difference between the amount of air supplied to said first zone and the amount of air supplied to said reducing zone for producing a third signal indicative of the excess air available to burn auxiliary fuel in said first zone;

means responsive to the lesser of the plant steam demand signal and said third signal for controlling the pressure of auxiliary fuel supplied to said first zone; means for summing said plant steam demand signal and a fourth signal indicative of the demand for air to partially burn the black liquor in the reduction zone to produce a fifth signal indicative of the total air required in the first zone; and

means responsive to a difference between said fifth signal and a sixth signal indicative of the actual amount of air supplied to said first zone for controlling said first zone outlet to provide said excess air to said first zone.

12. A unit according to claim 11, which further includes;

means responsive to at least a selected difference between the amount of air supplied to said second zone and the amount of air supplied to said reducing zone for producing seventh signal indicative of the amount of excess air available to burn auxiliary fuel in said second zone;

means responsive to the lesser of the plant steam demand signal and said seventh signal for controlling the pressure of auxiliary fuel supplied to said second zone;

means for algebraically summing an eighth signal indicative of the amount of air supplied to said second zone and a ninth signal indicative of the amount of air required in said second zone to completely burn the black liquor for producing a 10th indicative of the amount of excess air supplied to said second zone which is available to burn auxiliary fuel;

means for comparing said tenth signal to an llth signal indicative of the air required to burn the auxiliary fuel supplied to said second zone to produce a 12th signal indicative of the additional air required in said second zone to burn the auxiliary fuel; and

means responsive to the sum of the black liquor demand signal and the l2th signal for controlling said second zone outlet to supply sufficient air to said second zone to burn the black liquor and the auxiliary fuel. 

1. A recovery boiler for generating steam at a rate which varies with variations in a signal indicative of the total plant steam demand, said boiler including a reducing zone and at least one oxidizing zone for firing recoverable fuel and auxiliary fuel, which comprises: means responsive to the recoverable fuel firing demand for supplying primary air to said reducing zone; means for supplying said oxidizing zone secondary air in excess of the amount required to completely combust said recoverable fuel to establish an excess air condition therein; means responsive to said excess air condition for producing a secondary air available signal; means for supplying said auxiliary fuel to said oxidizing zone at a variable pressure; and means responsive to the lower of the secondary air available signal and the plant steam demand signal for regulating the pressure of the auxiliary fuel supplied by said auxiliary fuel supplying means to vary the rate of steam generation as the plant steam demand varies.
 2. A recovery boiler in accordance with claim 1 in which: said means responsive to said excess air condition is responsive to at least a preselected difference between the flow of primary air to said reducing zone and the flow of secondary air to said oxidizing zone for producing said secondary air available signal; and said regulating means includes means for measuring the regulated pressure of said auxiliary fuel, means for producing a feedback signal in response to a decrease of the regulated pressure below a selected minimum pressure, and means for adding the feedback signal and said lower signal to maintain the pressure of the auxiliary fuel at no less than the minimum pressure.
 3. A recovery boiler according to claim 1, in which: said secondary air supply means is responsive to both said demand for firing recoverable fuel and said plant steam demand so that in response to increased plant steam demand additional secondary air is supplied to said oxidizing zone to establish said excess air condition.
 4. A recovery boiler according to claim 1, in which: a plurality of oxidizing means are provided superadjacent said reducing zone in said boiler; said secondary air supplying means is responsive to both said plant steam demand and said demand for firing recoverable fuel for establishing said excess air condition in each of said oxidizing zones; and said secondary air available signal producing means and said pressure regulating means are provided for each of said oxidizing zones so that the pressure of auxiliary fuel supplied to each of said oxidizing zones varies as the plant steam demand varies.
 5. A system for controlling the combustion of auxiliary fuel in a black liquor recoveRy boiler, which comprises: boiler means for burning black liquor and auxiliary fuel, said means including a reduction zone and at least one oxidizing zone, said means including an air supply for each of said zones; means responsive to the plant steam demand for generating a plant steam demand signal; means for producing a black liquor demand signal indicative of the amount of black liquor supplied to said boiler means; means responsive to the sum of the plant steam demand signal and the black liquor demand signal for controlling the air supply for said oxidizing zone to establish in said oxidizing zone a condition of air in excess of that required to fully combust said black liquor; means responsive to said excess air condition for producing a signal indicative of the amount of said excess air; means for selecting the lower of the plant steam demand signal and said excess air signal to produce an auxiliary fuel control signal; and means responsive to said control signal for regulating the amount of auxiliary fuel supplied to said oxidizing zone.
 6. A system according to claim 5, in which: said regulating means includes means for measuring the pressure of the auxiliary fuel supplied to said oxidizing zone, means for comparing the measured pressure to a minimum operating pressure to generate a feedback signal in response to a measured pressure less than said minimum pressure, and means for summing said control signal and said feedback signal to maintain said pressure of auxiliary fuel supplied to said first combustion zone equal to or in excess of said minimum pressure.
 7. A recovery boiler combustion control system according to claim 5, in which: secondary and tertiary oxidizing zones are each provided with conduit means for distributing air from said controlled air supply, each of said oxidizing zones also being provided with said regulating means for supplying auxiliary fuel to each secondary and tertiary zone; means are provided for each of said regulating means for measuring the amount of auxiliary fuel supplied to each of said oxidizing zones to generate secondary and tertiary auxiliary fuel flow signals; means are provided for each of said conduit means for producing secondary and tertiary airflow signals indicative of the amount of air supplied to each of said oxidizing zones; means are provided responsive to a difference between said secondary airflow signal and said secondary gas flow signal for generating a secondary signal to control the airflow in said secondary zone conduit means; and means responsive to a difference between said tertiary airflow signal and said tertiary gas flow signal are provided for producing a tertiary signal to control the airflow in said tertiary zone conduit means.
 8. A system according to claim 5, in which: a secondary oxidizing zone is provided for burning black liquor and secondary auxiliary fuel; a tertiary oxidizing zone is provided for burning black liquor and tertiary auxiliary fuel; said air supply and air supply controlling means provide excess combustion air for both said secondary and tertiary zones; said excess air signal producing means produces a secondary excess air signal indicative of the excess secondary air and a tertiary excess air signal indicative of the excess tertiary air; one of said selecting means is provided for each of said excess air signals so that secondary and tertiary auxiliary fuel control signals are produced; and said regulating means includes separate means responsive to respective ones of said auxiliary fuel control signals for regulating the amount of auxiliary fuel supplied to the secondary zone and the tertiary zone.
 9. A black liquor recovery boiler having a plurality of combustion zones for firing black liquor at a relatively constant rate and auxiliary fuel at variable rates to generate steam for use in a paper plant having a variable total plant steam demand, which comprises: MEANS responsive to a first signal indicative of the total plant steam demand and a second signal indicative of the demand for firing black liquor for supplying to said boiler combustion air sufficient to complete the combustion of the black liquor and to fully combust the auxiliary fuel; supply means for individually feeding the auxiliary fuel to said zones at selected pressures; means responsive to the lesser of the plant steam demand signal and a signal indicative of the actual amount of air available in each of said zones to combust the auxiliary fuel for regulating said supply means to supply auxiliary fuel to each said zone at a selected pressure; and means for apportioning said combustion air to said combustion zones according to the actual amount of auxiliary fuel supplied to each of said zones.
 10. A boiler for recovering chemicals from black liquor according to claim 9, in which: said means for supplying secondary air includes means for summing a signal indicative of the demand for primary air and the plant steam demand signal to produce a secondary air demand signal, means responsive to the actual flow of secondary air for producing an actual secondary air signal, and means responsive to the difference between said secondary air demand signal and said actual secondary air signal for supplying said excess secondary air to said oxidizing zone.
 11. In a unit for recovering chemicals from black liquor and for producing steam in response to a plant steam demand signal, said unit including a combustion chamber having a reducing zone and first and second oxidizing zones, means for supplying to said reducing zone black liquor in response to a black liquor demand signal, means for supplying to said reducing zone air required to partially burn said black liquor, means for supplying auxiliary fuel to each of said first and second zones and means for supplying air to said first and second zones; said air supply means having an air inlet, a first zone outlet and a second zone outlet; said unit being included in a plant having a variable steam demand, the improvement in said unit which comprises: means responsive to the total flow of auxiliary fuel for producing a first signal indicative of the air required to burn the total flow of auxiliary fuel; means responsive to the flow of air to said first zone and to said reducing zone for producing a second signal indicative of the amount of air available in said second zone to burn auxiliary fuel; means for summing a signal indicative of the difference between said first and second signals with the black liquor demand signal for controlling said air inlet to make excess air available to said first and second zones; means responsive to at least a selected difference between the amount of air supplied to said first zone and the amount of air supplied to said reducing zone for producing a third signal indicative of the excess air available to burn auxiliary fuel in said first zone; means responsive to the lesser of the plant steam demand signal and said third signal for controlling the pressure of auxiliary fuel supplied to said first zone; means for summing said plant steam demand signal and a fourth signal indicative of the demand for air to partially burn the black liquor in the reduction zone to produce a fifth signal indicative of the total air required in the first zone; and means responsive to a difference between said fifth signal and a sixth signal indicative of the actual amount of air supplied to said first zone for controlling said first zone outlet to provide said excess air to said first zone.
 12. A unit according to claim 11, which further includes; means responsive to at least a selected difference between the amount of air supplied to said second zone and the amount of air supplied to said reducing zone for producing a seventh signal indicative of the amount of excess air available to burn auxiliary fuel in said second zone; means responsiVe to the lesser of the plant steam demand signal and said seventh signal for controlling the pressure of auxiliary fuel supplied to said second zone; means for algebraically summing an eighth signal indicative of the amount of air supplied to said second zone and a ninth signal indicative of the amount of air required in said second zone to completely burn the black liquor for producing a 10th signal indicative of the amount of excess air supplied to said second zone which is available to burn auxiliary fuel; means for comparing said tenth signal to an 11th signal indicative of the air required to burn the auxiliary fuel supplied to said second zone to produce a 12th signal indicative of the additional air required in said second zone to burn the auxiliary fuel; and means responsive to the sum of the black liquor demand signal and the 12th signal for controlling said second zone outlet to supply sufficient air to said second zone to burn the black liquor and the auxiliary fuel. 